Implantable medical devices are used to produce therapeutic results in a patient or for monitoring physiologic parameters of a patient. Examples of IMDs include implantable drug infusion pumps, implantable neurostimulators, implantable cardiovertor defibrillators, implantable cardiac pacemakers, and cochlear implants. Most of these IMDs either provide an electrical output or contain electrical circuitry to perform their intended functions. These devices are typically powered by a battery contained within the housing of the implantable medical device.
As the useful life of an implantable medical device is dependent upon the operating life of the battery that provides power, the development of rechargeable power sources that can be charged using electromagnetic energy from outside the patient's body provides the opportunity for longer life implantable medical devices. The ability to deliver electromagnetic energy to the charging circuitry within the implantable medical device is affected by the electrical characteristics of the housing of the implantable medical device. Typically, implantable medical device housings are made of a biocompatible metal such as commercial pure titanium. It has been suggested to employ a housing formed of a material having a higher resistivity than conventional commercial pure titanium to improve electrical performance of the recharging circuitry. During the recharge process, eddy currents can form in the housing due to the electromagnetic energy transmitted to recharge the battery. Because the eddy currents heat the housing, the amount of energy transferred to recharge is limited to prevent excessive heating of the device, resulting in relatively slow recharging of the battery. By employing a higher resistivity housing, the amount of energy that may be employed to recharge the device may be increased, and thus may shorten the time to recharge the device. A higher resistivity housing would also enhance telemetry to and from the implantable medical device, and would reduce magnetic resonance imaging (MRI) heating effects when a patient with an implantable medical device is subjected to an MRI procedure.
Decreasing the mass of conductive material in which eddy currents may be formed may also serve to improve the recharge and telemetry performance of an implantable medical device or reduce MRI-induced heating. However, problems may arise with housings that are too thin, as the structural integrity may be weakened and the ability to maintain a hermetic seal may be compromised.